A current sensor is a device that detects electrical current in a conductor and generates a signal proportional to the detected current. One type of current sensor suitable for the detection of alternating current (A/C) flowing in a conductor is known as a “current transformer.” A typical current transformer includes a split ring made of ferrite or soft iron with a wire coil wound around one or both halves, forming one winding of the current transformer, where the conductor carrying the electrical current being detected forms the other “winding.”
A current sensing switch or relay combines a current sensor with logic circuitry which controls a switch based upon a comparison between the generated signal and a threshold (a.k.a. “trigger” and “trip point”) level. When the threshold level can be manually adjusted, it is sometimes referred to as a “setpoint” because it has been set by, or on behalf of, the user. Current sensing switches can react to overload conditions and/or underload conditions, depending upon the type.
Traditional logic circuitry for current sensing switches tends to be analog in nature. Typically, a trimming potentiometer (“trim pot”) is used to adjust the setpoint to a desired level. While such circuitry is inexpensive it suffers from a lack of accuracy in that the trim pot tends to be only accurate in the ±20% range.
More sophisticated current sensing switches may use digital logic circuitry, such as a microcontroller, to provide more flexibility and accuracy with respect to setpoint settings. For example, the ASM Series Self-calibrating Smart-Relay of NK Technologies of San Jose, Calif. (“NKT Smart-Relay”) uses an actual load current as detected by a current transformer to set one or more trip points which are stored in digital memory. After a few seconds of steady running conditions, the NKT Smart-Relay's microcontroller locks onto the “normal” current level, after which automatically establishes trip points at 85% of the normal current level (for underload conditions) and 125% of normal current level (for overload conditions). The NKT Smart-Relay is self-powered by drawing power from its current transformer.
While current sensing switches such as the NKT Smart-Relay are flexible and useful for many applications, they are not well adapted to accurately setting specific setpoints. For example, the NKT Smart-Relay depends upon initial steady running conditions to automatically set trips points 25% above 15% below the “normal” current level. Since the “normal” current level can fluctuate, this can cause undesirable and unpredictable variances in the trip point levels and automatically builds in a large tolerance to a desired trip point. It is therefore difficult to accurately set specific setpoints values with conventional current sensing switches.